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The watch_directories() function takes a list of paths and a callable object, and then repeatedly traverses the directory trees rooted at those paths, watching for files that get deleted or have their modification time changed. The callable object is then passed two lists containing the files that have changed and the files that have been removed.

Python, 73 lines
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from __future__ import nested_scopes

import os, time

def watch_directories (paths, func, delay=1.0):
    """(paths:[str], func:callable, delay:float)
    Continuously monitors the paths and their subdirectories
    for changes.  If any files or directories are modified,
    the callable 'func' is called with a list of the modified paths of both
    files and directories.  'func' can return a Boolean value
    for rescanning; if it returns True, the directory tree will be
    rescanned without calling func() for any found changes.
    (This is so func() can write changes into the tree and prevent itself
    from being immediately called again.)
    """

    # Basic principle: all_files is a dictionary mapping paths to
    # modification times.  We repeatedly crawl through the directory
    # tree rooted at 'path', doing a stat() on each file and comparing
    # the modification time.  

    all_files = {}
    def f (unused, dirname, files):
        # Traversal function for directories
        for filename in files:
            path = os.path.join(dirname, filename)

            try:
                t = os.stat(path)
            except os.error:
                # If a file has been deleted between os.path.walk()
                # scanning the directory and now, we'll get an
                # os.error here.  Just ignore it -- we'll report
                # the deletion on the next pass through the main loop.
                continue

            mtime = remaining_files.get(path)
            if mtime is not None:
                # Record this file as having been seen
                del remaining_files[path]
                # File's mtime has been changed since we last looked at it.
                if t.st_mtime > mtime:
                    changed_list.append(path)
            else:
                # No recorded modification time, so it must be
                # a brand new file.
                changed_list.append(path)

            # Record current mtime of file.
            all_files[path] = t.st_mtime

    # Main loop
    rescan = False
    while True:
        changed_list = []
        remaining_files = all_files.copy()
        all_files = {}
        for path in paths:
            os.path.walk(path, f, None)
        removed_list = remaining_files.keys()
        if rescan:
            rescan = False
        elif changed_list or removed_list:
            rescan = func(changed_list, removed_list)

        time.sleep(delay)

if __name__ == '__main__':
    def f (changed_files, removed_files):
        print changed_files
        print 'Removed', removed_files

    watch_directories(['.'], f, 1)

This recipe is useful where you'd like some way to send jobs to a daemon, but don't want to use some IPC mechanism such as sockets or pipes. Instead, the daemon can sit and watch a submission directory, and jobs can be submitted by dropping a file or directory into the submission directory.

Locking is not taken into account. The watch_directories() function itself doesn't really need to do locking; if it misses a modification on one pass, it'll notice it on the next pass. However, if jobs are written directly into a watched directory, the callable object might start running while a job file is only half-written. To solve this, you can use a lockfile; the callable must acquire the lock when it runs, and submitters must acquire the lock when they wish to add a new job. A simpler approach is to rely on the rename() system call being atomic: write the job into a temporary directory that isn't being watched, and once the file is complete use os.rename() to move it into the submission directory.

2 comments

Zabil CM 10 years, 12 months ago  # | flag

Good one. I tried the script for windows and it worked like a charm. The only thing i did not get to work however was watch on mapped folder's/Network directories. Trying to figure that out.

Eli Criffield 8 years, 8 months ago  # | flag

inotify is similiar for linux 2.6.13+. if your using Linux 2.6.13+ there is a kernel interface for watching directory's and a python module for it at http://pyinotify.sourceforge.net/

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